Britney Spears finally checked herself into rehab, but her detox didn’t even last as long as her 55-hour first marriage.

The hard-partying mother of two checked out of a Caribbean rehab center after just one day, reports said.

“She refused to stay,” celebrity news service TMZ.com reported.

Spears apparently headed for rehab after leaving New York this week in a private jet with her sons – Sean Preston, 1, and Jayden James, 5 months.

People magazine online said the singer – who starred in the 2002 box-office bomb “Crossroads” – checked into Eric Clapton’s Crossroads Centre in Antigua, but checked out less than 24 hours later.

She flew coach back to Los Angeles from Miami yesterday on an American Airlines flight.

“She waved and smiled at fellow passengers,” one fellow traveler told The Post. “She didn’t order anything to drink. People were pretty much ignoring her.”

The passenger said, “Two guys were on the plane with her, and got her stuff and helped her get off the plane.”

Spears’ rep did not return an e-mail for comment. She denied the reports to TV’s Access Hollywood.

The 25-year-old has hit the party scene hard since splitting from dancer hubby Kevin Federline in November, and has been a paparazzo’s dream – partying with Lindsay Lohan and Paris Hilton and engaging in several public displays of drunkenness, vomiting, affection and her womanhood.

On New Year’s Eve, she apparently passed out drunk at a Las Vegas club, but later said she’d simply fallen asleep.

The former Mouseketeer was a one-woman party whirlwind on her recent trip to New York, hitting numerous clubs until all hours. Her more memorable stops included upchucking at Tejune last Saturday night, and trading in her dress for a dancer’s bikini and busboy coat at Meatpacking District hotspot One Little West 12th on Sunday night.

“She’s in total denial about her problems. She kept wanting to go out,” a source who spent a lot of time with her in the Big Apple said.

“You can’t say anything to her. She’s never heard the word ‘no.’ It’s always, ‘OK, Britney,’ and if not, she’ll go get a new assistant or new friends who will follow her around.”

Family and friends have pleaded with her to clean up her act. Her long-time assistant Felicia Culotta, who recently quit, even went so far as to post a public plea on the gossip site ThatOtherBlog.

“All I can do is be a friend . . . and PRAY,” Culotta wrote. “I cannot save her from herself, nor can I commit her to any type of treatment program against her wishes and will. I am throwing up my hands and realizing that I am helpless.”

Culotta said Britney’s not hopeless, but has to sink lower before she climbs back up. “When she does indeed hit rock bottom, she’ll stand up and walk away from this whole fiasco a new, confident, changed, career-driven Britney like we all knew and loved,” she wrote.